Water & Wastewater Asia March/April 2022

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FOCUS EVAPORATION TO MAKE the digestate from anaerobic digestion reusable The reuse of digestate from anaerobic digestion as water for different purposes within a plant. By Cristina Del Piccolo A plant in Belgium feeds an anaerobic codigestion plant with the organic fraction of municipal solid waste (OFMSW), kitchen waste, waste from supermarkets and floated sludge. The plant is composed of a pulper to pre-treat part of the waste and two anaerobic reactors that produce biogas in mesophilic conditions. The by-product of the reaction is wastewater composed of digestate after solid-liquid separation and a smaller stream from the pulper. The wastewater is highly polluted, with a high load of organic residues, suspended solids and salinity. The plant wants to be virtuous and reduce tap water consumption, hence, it puts in place an approach aiming at increasing the recovery and reuse of water. There are several points in the plant where water is required. Furthermore, the quality for reused has to be the same and be at the maximum. He started from an analysis of the qualities of water he had available and at the beginning, when the plant was put in operation, they were two: • A very raw one, that came just after a solid-liquid separation and that was considered suitable for the pulper, • An intermediate one from the aerobic biological plant so with reduced pollution in terms of organics and ammonia which was considered suitable for the anaerobic reactors. For the uses such as biofilters, scrubbers and polymer preparation, the operator, in the beginning, was using tap water from the municipality, since the two streams from reuse he had implemented till that time were considered good enough. To further reduce the income of tap water and increase the percentage of water reused inside the plant, the operator decided to add a treatment stage to polish the wastewater to a very high quality so that it could substitute the need for tap water and make the plant independent from an external source. The treatment stage that was added to accomplish this task was an Evaporator from Evaled series AC F fed with hot water and working with forced circulation to manage highly concentrated and polluted streams. Evaporation technology is already part of the solution when highly charged effluents need to be treated. There are several examples of evaporation plants treating successfully landfill leachate which is wastewater quite similar to the liquid fraction of the digestate. The evaporator was also successful in reducing the salinity that was circulating in the plant and that was increasing due to the closed-loop the operator was realising in the plant. The other advantage that the evaporator brought was to recover the thermal energy from the biogas cogeneration plant, otherwise lost, as thermal energy to sustain the evaporation process so that the Opex associated with the new stage is significantly reduced. In the evaporation process, the main contribution to the Opex is given by the energy consumption: If like in this case, it is possible to take advantage of an available free source otherwise lost, the advantage in terms of reduced Opex is immediately evident. The Evaled evaporation stage is composed of: • A pre-treatment which is essentially a pH adjustment and a degassing of the CO2 developed during the acidification phase. The main purpose is to reduce scaling by the elimination of alkalinity always present in such liquid. • A main treatment which is based on evaporation technology with forced circulation. The digestate is sucked inside the boiling chamber of the evaporator Evaled AC F 40 FF by the vacuum created inside it. The vacuum is created by the liquid ring pump coupled with the ejector that works with the Venturi principle using air as the motor fluid. A circulation pump sends the liquid from the bottom of the boiling chamber inside the shell and tube heat exchanger where it receives the necessary heat from the hot water stream that circulated inside the heat exchanger. Once heated, the liquid returns to the boiling chamber, and as a result of the vacuum, a portion of the liquid immediately boils. The vapour rises through the demister to dampen the droplets. Vapour is condensed against the U tubes of the heat exchanger. The steam condensation is then performed by an air cooler connected to the evaporator which provides and circulates the cold water for the condensation. 34 WATER & WASTEWATER ASIA | MARCH/APRIL 2022
FOCUS 34-35 tons/day of distillate as pure water and about 3-4 tons/day of concentrate recovered as compost after composting. The distillate produced by the evaporator is reused internally in the plant that requires the highest quality. The concentrate is sent to the composting plant to be mixed with the solid fraction separated after the anaerobic stage and transformed into compost which is then sold as fertiliser to farmers, realising a completely closed and highly valuable circular approach. The vacuum system extracts the condensed distillate together with any non-condensable gases and sends them to a storage tank. The distillate and the concentrate are discharged through a pump. All operations are controlled by a PLC. The heat exchange is effectively assured by the forced circulation principle, meaning that the liquid is transferred by a centrifugal pump which imposes a certain velocity so that scaling and fouling are prevented to happen on the surface of the tubes of the heat exchanger, thus assuring the possibility to get the highest recovery combined with the lowest maintenance possible. It is the preferred choice, especially in wastewater treatment and especially when the streams to treat are highly polluted, with high concentrations of organics and other compounds close to solubility which is reached later as long as concentration and recovery increase to assure smooth operation. steels which ensure high tolerance to anions such as chlorides, sulfates and more in high concentrations. The hot water comes from the co-generation plant that transforms the biogas produced by the anaerobic co-digestion process upfront into electrical energy which is sold. The engine of the co-generation plant requires to be cooled down and this generates a hot water stream which is normally not recovered but that in this plant is considered a precious resource and sustains the evaporation process in the Evaled unit. An overview of the Evaled evaporation stage The Evaled AC F 40 FF unit works with a recovery of about 90%, being fed with about 38 tons/day of digestate, producing about The evaporation plant has been successfully in operation for almost 20 years and has been upgraded with a newer unit to increase the evaporation capacity and be aligned with the latest increase in treatment capacity of the overall waste treatment plant. The inclusion of the evaporation stage in this complex waste treatment plant allowed the operator of the plant to achieve several goals, including: • Zeroing of tap water need and income, making the waste treatment facility 100% autonomous and independent from an external water supply. • Achievement of a closed-loop circuit and reduction of the environmental impact of the waste plant. • Optimisation of the available sources and utilities through the recovery of thermal energy previously lost, to feed the evaporation process for free. Cristina Del Piccolo is CTO of Veolia Water Technologies Italy. The experience in treating highly polluted wastewaters is also evident in the choice of the material of construction of the items in contact with the concentrate – the digestate is typically salty wastewater and so the material has to be corrosion resistant. For this reason, Evaled proposes super-duplex stainless The anaerobic co-digestion process WATER & WASTEWATER ASIA | MARCH/APRIL 2022 35
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